1. Field of the Invention
The present invention relates to a pigment-dispersed aqueous ink composition suitable for an ink jet printing system.
2. Related Art
Dye-type ink compositions basically consisting of a dye and an aqueous medium have been known as inks for ink jet printing. It has been pointed out that images produced by the inks of this type are poor in both light resistance and water resistance because of the inherent properties of the dye. In order to solve this problem, inks prepared by using, instead of a dye, a pigment have been proposed.
In the meantime, inks for ink jet printing are required to have the following properties: to undergo no change in the physical properties during long-term preservation; not to cause obstruction in a fine nozzle; to produce sharp images with a high density; and to produce images having high preservability (water resistance, light resistance, etc.). In particular, pigment-type inks are required to have high preservability, i.e., the ability to stably maintain the pigment in a dispersed state for a prolonged period of time, and not to cause obstruction in a nozzle while printing is conducted or when the printer is restarted after interruption of printing.
The following have been proposed as pigment-type inks:
Japanese Patent Publication No. 1426/1987 discloses an ink which is prepared by dispersing a pigment and a resin emulsion in water. According to this, the preservability of a pigment-type ink can be improved by adding thereto an emulsion of a water-insoluble synthetic resin. However, the resin contained in the ink tends to solidify at a nozzle tip to cause obstruction.
Japanese Laid-Open Patent Publication No. 157668/1980 discloses that when a pigment is dispersed in a water-insoluble resin emulsion, the dispersion can be stably maintained even when the viscosity of the dispersion is relatively low. Furthermore, Japanese Laid-Open Patent Publication No. 217088/1989 discloses that the preservability and the fluidity of an ink can be improved by using an emulsion having a specific film-forming temperature. However, even such inks are still insufficient in the stability at low- and high-temperature regions and cause obstruction in a nozzle.
Inks prepared by using a resin emulsion like the above are disclosed also in Japanese Laid-Open Patent Publications Nos. 160068/1991 and 18462/1992. There is however room for further improvement in the stability of these inks.
Japanese Laid-Open Patent publication No. 18427/1990 describes that inositol is added to an ink so that the ink will not cause obstruction in a nozzle. However, since inositol has hygroscopicity, this ink is poor in moisture-abrasion resistance (that is, when the surface of an image produced by this ink is touched by a hand or a finger, the image is abrased, or the hand or the finger is stained with the ink).
It is also pointed out that a pigment-type ink for ink jet printing has the problem of unevenness of printing, which has not been cased by a dye-type ink. Such an unevenness is remarkably found in a solid filled area, like a diagram or a bar graph. To solve the problem of unevenness of printing is an important subject in the art.
We have tried to improved the unevenness of printing by increasing the concentration of pigment in an ink and discharging the ink in excess, but failed. Thus, it seems that the unevenness of printing depends on composition of an ink composition.